18

Barbados

Barbados stands apart from the other large West Indian Islands in several respects: geographically, in being some 200 miles to the east of the string of islands known as the Lesser Antilles; geologically, in being a coral upheaval while the others are, at least so I am informed, volcanic peaks; entomologically, in being the only island free from Anopheles; and medically, in being the only island free from endemic malaria. The connection between the absence of Anopheles and the absence of malaria is easily understood, but why Anopheles should be absent is not quite clear. It has been suggested that the island’s geographical isolation has prevented the insect spreading to it from the other islands, but the generally accepted view is that it is due to the presence of a small minnow called the “millions.” I am not able to accept this view, and am inclined to think there is some connection between the absence of Anopheles and the geological structure of the island.

Although the inhabitant of Barbados is not pestered by Anopheles, he is the victim of several other mosquitoes which breed in tins, cisterns, barrels, and water containers of that sort, and the name “Barbados leg” (elephantiasis) shows these other mosquitoes to be present in sufficient numbers to give the island a distinction of an unenviable character.

In 1905, Mr C. Kenrick Gibbons in a letter to the West Indian Committee Circular suggested that the absence of malaria was due to the presence in the streams and ponds of the “millions.” These small fish are voracious feeders on the eggs, larvae, and pupae of mosquitoes, and it was thought that they destroyed all Anopheles in the ponds and streams; but that, since they could not reach Culex and Stegomyia larvae in artificial collections of water, these lived and flourished. Following on this suggestion, “millions” were sent to other islands; but, so far, I have not heard of any definite experiment showing a reduced spleen rate consequent on the introduction of the minnow.

Whatever the truth about the fish be, it is of importance that some definite experiment be undertaken. One of the smaller malarious islands should be stocked with “millions.” It is not enough that “millions” be placed in a few streams, every stream and pool in either a half or the whole of an island should be well stocked with the fish. Malaria statistics should be carefully collected, and the other half of the island, or another island as the case may be, should be chosen as a control. This is an experiment which would cost almost nothing, while its importance is great.

Although I urge that a definite experiment be undertaken—for in a problem so great as that of malaria prevention we should try all things and assume nothing—my impression is that the absence of malaria from Barbados is not so much connected with the presence of “millions,” as with the geological structure of the island. Having heard that the island was composed mainly of a porous coral rock, I spent the short time at my disposal in seeing to what extent breeding places for Anopheles existed.

One of the first things that strikes a visitor to this tropical island is the absence of jungle. There is a large population on the island, and it is cultivated from end to end. Taking a motor car I ran some miles up the west coast in order to cross the streams which figure so freely on the map. Except at one spot where there was a foul puddle under a bridge, all the stream beds were dry; while all around were wide stretches of gently sloping fields, obviously dry. At one point turning down to the sea, I came to what appeared to be a small piece of level ground, seemingly a raised coral reef or beach, and here there was a ditch containing water and crowded with “millions.” Returning towards the town, and crossing what on the map appears to be a considerable river, I found it to be dry also; and it is evidently dry at all times except during an actual rainstorm, for there was no sign of aquatic vegetation in its bed.

I then ran along the south coast, going out on a road well above the sea level, and returning by the shore road. Everywhere on the upper road the land was dry. The road surface, indeed, consisted simply of the soft coral rock. The rock had never been under pressure, and the branching of the coral was plainly visible in the roadside “cuttings.” This portion of the island, therefore, consisted of a porous coraline rock, rather than a limestone such as we see in the English Downs, or Malay limestone formation. The shore road was on a ledge which appeared to be a coral reef or raised beach some 10 to 15 feet above sea level. Near to Worthing, about three miles from Bridgetown, a low-lying portion of the ledge formed a swamp of roughly 10 acres in area, divided up into canals and paths like much of the land seen in British Guiana.

I understand that in a small portion of the east coast, St Andrew’s Parish, clay and sandstone appear, and that there the streams exist through the year; but I had no opportunity of visiting that portion of the island. I admit frankly that my visit was so brief that my views have not the weight which a longer stay might have given them; but being accustomed to visit places with the express object of detecting Anopheles breeding places, and having acquired a sufficient knowledge of country to know where to find water when it appears on the surface of the land, I am not disposed to think I can have overlooked many breeding places; indeed, in country so open as the portion of Barbados that I visited, the task of finding breeding places was peculiarly easy. One looks for water at the foot of a slope, where it forms a spring, or in river courses which convey the accumulated water of many springs. On stretches of gently sloping hill land such as forms the south and west of Barbados, water will not be found, unless some impervious stratum forces it to the surface. Now in Barbados, the only surface water I could find was a small puddle in a river bed and two water collections on level portions of the island. As these level portions are probably raised coral reefs, it is possible water will be found on other portions of old beaches; but when I consider the acres of dry land, and the scattered and limited nature of the surface water, I cannot believe that malaria is absent from the portion of Barbados that I saw, for any reason other than the almost complete absence of breeding places.

In the brief allusion to Barbados, which I made in 1913,89 time did not permit me to explain in full my reasons for doubting if the “millions” were entitled to all the credit they got. I do not deny that “millions” and many other fish and insects eat mosquito larvae, and had it been suggested that some of the trenches in British Guiana were free from Anopheles because of the “millions” and their allies, I should have hesitated to dissent; but to bring Barbados in support of the “millions” claim seems to me peculiarly unfortunate, since the possible Anopheles breeding places are so few and widely scattered. Barbados seems to me free from malaria because of the relative absence of breeding places, and to my mind that is because Barbados has natural subsoil drainage. Some such idea, too, may have been in the mind of the late Sir Rubert Boyce when he wrote:90

We now know that the malaria-carrying mosquitoes, the Anophelines, appear to be absent from Barbados; most probably this is due to the fact that suitable conditions for their development do not exist. Owing to the nature of the soil, the storm water is rapidly carried off through the innumerable “sucks” which are everywhere to be found in the porous coral rock, and what permanent pools do exist seem to be perpetually stocked with minute fish—the millions which effectively get rid of any mosquito larvae.

If we assume that the actual area of water is sufficient to allow of malaria becoming established in the island, even then it does not follow that the “millions” are the cause, or even an important factor in the causation of the absence of Anopheles. Dr G. C. Low is inclined to regard the isolation of the island as important. It is 200 miles to the east of the other islands, and that means 200 miles against the prevailing north-easterly trade wind. The chance of mosquitoes arriving by sailing vessels is therefore remote; and if they arrived by steamers, which lie out in the open roadstead, they are still half a mile from the shore. Human carriage, therefore, does not favour the spread of the insect.

Even if a boatload of sturdy Anopheles were landed at Bridgetown and hastened to quarter themselves on the inhabitants, it is difficult to see how they are to propagate their species. The chances of any pregnant insect finding a breeding place a few miles out of the town, as for example the swamp at Worthing, and there can be few nearer, are as remote as those of their landing at all.

Finally, supposing Anopheles did land and did find a breeding place, is it certain that the fish is the cause of the mosquitoes’ failure to thrive? Dr Low mentioned in the discussion which followed my paper, that he had found Culex and other larvae in the swamp at Worthing, just as he had noticed Anopheles larvae in Italy in waterways teeming with fish. I have seen many instances of the same in many lands, and have referred to the subject specially in this book in connection with a ravine in Sumatra, a swamp at Gatun in Panama, and a trench at Plantation Port Albion, in British Guiana. Dr Low’s observation of the existence of the mosquito larvae and “millions” in the swamp at Worthing in Barbados, which appears to have been entirely overlooked by those who advocate the “millions” theory, is the last and final blow to the theory; for if “millions” are so voracious and so deadly to mosquito larvae in a pond, why do Culex larvae escape while Anopheles are utterly destroyed?

In searching for a reason for the absence of Anopheles from the Worthing swamp, Dr Low suspected that the chemical constitution of the water was unsuitable for the larvae, but on taking some of the water to another island he found Anopheles larvae bred freely in it. This test is, however, of little value, for the larvae of A. albimanus, with which Dr Low experimented, will breed out even when put into pure sea water.91

To sum up, it seems to me that (a) the general freedom of Barbados from malaria is due to the practical absence of surface water owing to the geological structure of the island; (b) that such ponds as exist are free from Anopheles, yet contain Culex larvae, for the same reason as certain trenches in British Guiana; and (c) that this is connected with the high state of cultivation in the island, the absence of jungle, and other, at present unknown, conditions governing mosquito life.

Again arises the question of why some mosquitoes breed in one class of breeding place and others in another; why some rice fields are free from Anopheles and malaria, and others full of both; and why it is possible to alter the species of Anopheles found in some places by altering the conditions of the breeding place, e.g., by drainage and clearing jungle. And again I would insist on the urgent need for a detailed study of the whole biology of the mosquito, for by this means I am convinced we will attain such knowledge as will give us power to say to some species “Come” and to others “Go,” and will extend our control over malaria in a way that at the present time is almost beyond our dreams. But since in our present state of darkness we should neglect nothing, I strongly urge a definite scientific experiment with “millions” in one of the malarious West Indian Islands.